Photovoltaic power represents a non-depletable resource which is globally available and non polluting. Because of the increasing scarcity of non-renewable energy sources such as coal, petroleum and uranium and the ever-increasing problems attendant upon their use, it is essential that greater use be made of solar energy.
Single crystal photovoltaic devices, especially crystalline silicon photovoltaic devices have been utilized for some time as sources of electrical power. However, the utility of such crystalline devices has been limited by problems associated with the manufacture thereof. More particularly, single crystalline materials are difficult to produce in sizes substantially larger than several inches in diameter, relatively fragile, relatively thick, and heavy; furthermore, they are expensive and time consuming to fabricate. Recently, considerable effort has been expended to develop systems and processes for preparing thin film semiconductor alloy materials which encompass relatively large areas and which can be deposited so as to form doped semiconductor layers for the production of thin film photovoltaic devices which are substantially equivalent, or superior, to their crystalline counterparts in operation and efficiency. Such materials are disclosed in U.S. Pat. Nos. 4,226,898 and 4,217,374 of Ovshinsky et al. It is now possible to deposit high quality, thin film semiconductor alloy materials over large areas in a roll-to-roll process so as to enable the fabrication of large area devices. Such techniques are disclosed in U.S. Pat. Nos. 4,410,558 and 4,485,125. Such large area deposition techniques offer the advantage of high speed and economy. Generally, it is desirable to subdivide a large area photovoltaic device into a plurality of smaller area devices which are interconnected in series and/or parallel arrays to provide a desired level of voltage and power.
Subdivision and interconnection of a large area device may be carried out by severing the large area device into a plurality of discrete devices which are then interconnected to form a module. This technique is quite labor intensive and more practical for forming modules from a relatively smaller number of large devices. However, it is impractical for interconnecting a large number of small area devices. Another approach to the task of interconnecting small area devices involves the manufacture of monolithic devices. As utilized within the context of the present invention, the term "monolithic" defines an interconnected array formed from a body of semiconductor material which is disposed upon a single large area substrate; in contrast to the discrete cell approach, fabricating a monolithic device involves no severing of the substrate.
Techniques for the manufacture of monolithic arrays of photovoltaic devices have been implemented in the prior art and such techniques typically involve the use of etching, scribing or similar techniques for subdividing semiconductor layers into discrete, electrically isolated portions and employ subsequent steps for depositing additional semiconductor layers and for interconnecting these portions. Prior art techniques generally involve scribing a device into small area cells which are connected in a top to bottom relationship. These techniques necessitate alternating vacuum deposition techniques with scribing or wet-etching or plasma etching steps. The repeated steps of vacuum deposition and atmospheric processing contaminate both the deposition apparatus and the devices, thus reducing the quality of the devices and the efficiency of the process.
In typical prior art techniques, contact between the isolated subcells is generally made between relatively large area portions of adjoining cells. The reason for the large area contact is two fold. Generally, the subcells are fairly small and a large contact area is utilized to accommodate various inaccuracies of the screen printing or lithographic process utilized for interconnection; also, contact is generally made through the use of printable materials such as electrically conductive paste or inks and these materials typically manifest a fairly high contact resistance; consequently, a large area must be utilized to enable full withdrawal of photogenerated power. This large contact area represents a loss of active cell surface. U.S. Pat. No. 4,315,096 of Tyan et al. discloses a technique for fabrication of an array of photovoltaic devices, which technique utilizes a contact region extending across the width, and a substantial portion of the length, of the subcells of the device. Other techniques for the fabrication of interconnected arrays are disclosed in U.S. Pat. No. 4,754,544.
The preparation of most thin film photovoltaic devices involves depositing various semiconductor layers by at least one vacuum step employing a process such as evaporation, sputtering or glow discharge deposition. The presence of volatile species on the device or substrate can contaminate the interior of a vacuum deposition apparatus and degrade the semiconductor layers. Many prior art techniques for the preparation of monolithic photovoltaic devices require vacuum deposition of semiconductors to be carried out on a device which has previously been subjected to wet etching, solvent based coating or other such techniques which can leave a residue of volatile contaminants thereupon. Since prior art techniques require intermingled deposition and etching steps, the semiconductor deposition cannot be separated from the array fabrication. It is not possible to employ a "generic" photovoltaic body which is amenable to various processing techniques. It clearly would be desirable to have a method for manufacturing a monolithic photovoltaic device which separates the semiconductor deposition steps from the wet processing steps.
Prior art interconnection techniques also tend to give a high series resistance array. The interconnections between the cells often rely upon the use of materials which alloy with the various cell components to create a high resistance contact. In other instances, the processing techniques oxidize or otherwise degrade the contacts. It is therefore desirable to interconnect small area subcells by a technique which avoids oxidation or alloying and thereby provides a low resistance contact.
The present invention provides an improved monolithic photovoltaic device comprised of a plurality of interconnected small area subcells disposed upon a single, large area substrate. The connections between the cells are made through the use of materials which retain high conductivity. Thus, contact areas can be made small. The small contact area also provides increased active cell area and hence high photo conversion efficiencies. The device of the present invention is manufactured by a process which segregates the semiconductor deposition and wet processing steps so as to avoid problems of contamination and to speed up process time. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.